Alkali metals are employed as a component of ultraviolet band-pass filters. In such filters, a thin film of the alkali metal is used as a filter to achieve optical characteristics necessary for far ultraviolet imaging, particularly for use with silicon CCD sensors. The alkali metals are employed because they achieve a broad far ultraviolet band-pass, yet provide substantial rejection of wavelengths longer than 3000 .ANG.. Further, alkali metal filters achieve high optical surface quality and environmental stability within a compact physical format appropriate for use in a filter wheel. Alkali metal filters are ideally suited for numerous applications requiring far ultraviolet filtering.
A conventional alkali metal filter is formed by depositing a layer of an alkali metal such as sodium onto a substrate window by vapor deposition. To achieve an optimal filter, the alkali metal layer must be physically stable, otherwise optical surface quality may degrade. When initially deposited, the alkali metal typically has a crystalline structure. Unfortunately, the crystalline structure of the alkali metal layer is unstable and surface migration can occur on an outer surface of the alkali metal film. Such surface migration is caused by the high surface free energy. The alkali metal atoms cluster into thick areas and voids, resulting in a general nonuniformity of the alkali metal layer which degrades the optical performance of the filter.